Process for inhibiting corrosion and product



Dec. 13, 1960 INHIBITING VALUE H. T. FRANCIS ETAI. 2,964,433

PROCESS OR INHIBITING CORROSION AND PRODUCT GOOD FAIR

POOR

I.OM.

ZnClz Filed Dec. 13, 1957 WHITE RUST INHIBITION BY SPRAYED MIXTURES OF ZI1CI CV0 INCREASING CF0 0.5M I OM.

ZI'I CI2 CI'O 3 Cr 0 3 INVENTORS HOWARD Z' FRANCIS FRANK H ROEBUGK ATTORNEYS United States Patent" PROCESS FOR INHIBITING CORROSION AND PRODUCT Howard T. Francis, Park Forest, and Frank H. Roebuck, Chicago, Ill., assignors, by mesne assignments, to National Steel Corporation, a corporation of Delaware Filed Dec. 13, 1957, Ser. N 0. 702,659

27 Claims. (Cl. 148-'-'6.2)

This invention relates to a process for treating metals to retard corrosion, and more particularly, to a novel process for treating material having a zinc surface to inhibit white rusting and generally improve corrosion resistance. The invention further relates to the novel corrosion inhibited product produced by the process described herein. 7

The invention will be described and illustrated hereinafter with specific reference to a process for inhibiting white rust formation on materials having a zinc surface. However, it will be recognized by those skilled in the art that the process of the present invention also may be useful in retarding or inhibiting other types of corrosion and generally improving corrosion resistance. The term zinc surface as used in the specification and claims is-intended to include a surface of a material subject to white rusting which is composed of zinc or predominantly of zinc. Examples of materials having azinc surface subject to white rusting and suitable for treatment in accordance with the present invention include metallic forms or articles composed of zinc or predominantly of zinc, base metals such as ferrous metals or articles fabricated therefrom which are provided with a protective coating of zinc or zinc alloys containing a predominant amount of zinc, etc. It is understood that a suitable material for treatment may have a zinc surface as herein defined on only a portion of its surface area, and that such materials may be treated in accordance with the invention for the purpose of improving corrosion resistance of at least that portion of the surface area provided with the zinc surface.

A fresh, bright, untreated zinc surfaced material exposed to the atmosphere soon develops a surface film composed of corrosion products Which'are'producedby the action of atmospheric substances such as carbon dioxide, oxygen and moisture. As corrosion proceeds, the surface film tends to increase somewhat in thickness and the layer of corrosion products becomes a white deposit which renders the appearance of the zinc surface less pleasing to the eye. However, a zinc surfaced material directly exposed to weather is generally capable of giving long service without great loss of its appearance value since thin films of the corrosion products are more or less continuously removed by the elements, thereby preventing an appreciable build-up in the thickness of the layer of deposited corrosion products and leaving the zinc surface more or less uniform in appearance.

White rust is a term commonly used in the art to refer to a specific form of the above mentioned type of corrosion. It is generally considered to be a relatively thick white deposit of corrosion products composed largely of zinc hydroxide and basic zinc carbonate which forms on unprotected zinc surfaced materials exposed to air and excessive amounts of moisture or water. The development of white rust occurs most rapidly when Water is confined against the zinc surface by an overlying. member, and it is particularly severe where fresh,

bright, untreated zinc surfaced sheets or shapes are arranged during storage or shipment in such a way that water can accumulate between adjacent surfaces and remain for extended periods of time. For example, galvanized sheets useful in the construction of metallic buildings are commonly stacked in open railway cars, either packaged or Without packaging, and shipped over long distances, or stored for long periods of time awaiting shipment or use. Water accumulates between adjacent sheets during such periods due to sweating and white rust quickly forms on those portions of the sheets directly exposed to the water. Over a period of time, the coating of white rust gradually thickens and eventually the appearance of the originally bright, spangled galvanized surface is completely destroyed and it takes on the appearance of an inferior product.

The art has long sought a suitable process for treating zinc surfaced materials to effectively inhibit white rust formation. In general, the prior art processes involve the application of either a relatively thick protective coating such as oils, waxes, greases, varnishes, paints, etc., or various chemical treatments which deposit a thin protective coating integral with the zinc surface. In every instance, the prior art processes are unsatisfactory due, for example, to their being ineffective in substantially completely eliminating white rust formation for a reasonable period of time, too expensive, too time consuming for use in high-speed continuous galvanizing operations or the treatment results in a protective coating which must be removed prior to use of the treated zinc surfaced material. Also, in some instances, the coating substance either destroys or mars the appearance of the originally bright pleasing finish of the fresh zinc surface. The prior art chemical treatments, in particular, require careful control of operating conditions such as concentration of ingredients and temperature of treatment, and often exhibit a pronounced tendency toward formation of undesirable colored films on the treated zinc surface.

It is an object of the present invention to provide an improved process for inhibiting corrosion of zinc surfaced materials which overcomes the above mentioned disadvantages of the prior art processes.

It is still a further object of the present invention to provide an improved process for producing a corrosion inhibiting film on materials having a zinc surface by intimately contacting a hot zinc surface with effective amounts of a solution in a volatile solvent of at least one suitable substance which is a source of zinc ion and :at least one suitable substance which is a source of chromate ion.

It is still a further object of the present invention to provide material having a zinc surface which is treated in accordance with the process of the present invention wherein the treated zinc surface is effectively inhibited against the formation of white rust and is not discolored or changed substantially in appearance.

Still other objects of the present invention and the advantages thereof will be apparent to those skilled in the art by reference to the following detailed description and the drawing, which graphically illustrates the white rust inhibiting value of atomized aqueous solutions of zinc chloride and chromic acid.

In accordance with the present invention, materials having azinc surface are treated for the purpose of depositing a corrosion resistant film on the zinc surface and thus improving corrosion resistance by intimately contacting the zinc surface with a solution in avolatile solvent of at least one suitable substance which is-a source of zinc ion and at least one suitable substance which is ion is about 9:1 to 1:9.5.

a. source of chromate ion under conditions to be fully described hereinafter.

It is essential that the inhibitor solution be intimately contacted with the zinc surface, and that the zinc surface to be contacted with the solution be initially at an elevated temperature substantially above the boiling point of the particular volatile solvent used and below the melting point of zinc. For example, when water is the solvent, the initial temperature of the zinc surface to be contacted with solution should be from about 225 F. to about 600 F. In addition, the initial temperature of the zinc surface to be contacted with solution must be sufficiently hot so as to volatize the volatile components present in the volume of solution which is intimately contacted with the surface substantially immediately after the instant of contact and to deposit a corrosion inhibiting film. When water is the solvent, an initial temperature below about 225 F. is not sufliciently elevated to accomplish the foregoing and to give generally satisfactory results; while at an initial temperature above about 600 F., the rate of evaporation is generally such as to prevent or to not allow sufficient time for intimate contact between the solution and the zinc surface before complete volatilization of the volatile components. Thus, in most instances, a proper degree of inhibition of white rusting is not imparted to the zinc surface under such conditions. Generally, better results are obtained when water is the solvent at an initial surface temperature between 250 F. and 350 F.

The inhibitor solution must be in intimate contact with the zinc surface before evaporation of the volatile components of the solution is complete. While the volatile components of the inhibitor solution are volatilized almost immediately after the solution is intimately contacted with the hot zinc surface, nevertheless a short period of time expires between the instant the solution is intimately contacted with the surface and complete volatilization of the volatile components, and this short period of time is necessary in order to achieve acceptable results. Thus, when operating at relatively high temperatures, steps must be taken to assure that at least a portion of the solution is being deposited in intimate contact with the zinc surface rather than the volatile components being volatilized completely while enroute from the source of spray to the zinc surface and thereby resulting in the surface being contacted with only dry particles of substances which were dissolved in the solution. In view of the foregoing, it may be advisable when operating at high temperatures to increase the volume of solution applied to a given surface area and decrease the concentration of the solution; or reduce the distance between the zinc surface and the atomizing device, i.e., reduce the distance of travel of the atomized solution; or increase the size of the individual droplets in the atomized spray, thereby decreasing their rate of evaporation; or lower the temperature of the air space through which the atomized spray travels while enroute from the spray source to the zinc surface, thereby decreasing the rate of evaporation.

The inhibitor solution of the present invention is a solution in a volatile solvent of a substance which is a suitable source of zinc ion and a substance which is a suitable source of chromate ion, and may be prepared by dissolving one or more compounds capable of providing at least one of such substnces in the solvent. If either zinc ion or chromate ion is not present in the solution, then the degree of corrosion inhibition imparted to the treated zinc surface is not satisfactory. The ratio of zinc ion to chromate ion in the solution may vary over wide ranges. For example, by reference to the drawing, it may be seen that the treated corrosion inhibited product will exhibit a corrosion inhibition value of Good provided the ratio of zinc ion to chromate It will also be noted that superior corrosion inhibition will be imparted to the product if the ratio of zinc ion to chromate ion is about 4:1 to 1:9. In most instances, for best results the inhibitor solution preferably should contain about equal concentrations of zinc ion and chromate ion.

The inhibitor solution may contain as solutes a suitable soluble zinc salt and a suitable soluble source of chromate ion. However, the anion of the zinc salt and the cation of the chromate preferably should be volatile under the conditions of the process of the invention. For example, when water is the volatile solvent, suitable zinc salts include zinc chloride and zinc nitrate and a suitable source of chromate ion is chromic acid.

In addition, the solution should be acidic but inasmuch as suitable solutes often provide a proper level of acidity, the presence of free mineral acid may not be necessary or desirable. For example, when chromic acid is the source of chromate ion, then the resulting acidic solution does not require the presence of free mineral acid. Since acidic solutions of chromate are generally considered to contain both chromate and dichromate, and perhaps other species of chromate, it may be necessary to make adjustments in concentrations where the concentration of chromate or chromate ion is referred to in the specification and claims. Where the concentration of chromate or chromate ion is referred to, it is understood to mean the gram ion concentration of Crop which would be present in the solution if dichromate, or other species of chromate, were converted to Q0; '2 Thus, a solution containing 1.0 gram ion of dichromate would be considered to contain 2.0 gram ions of chromate or chromate ion for the purposes of the present invention.

The degree of white rust inhibition imparted to the treated surface is largely determined by the amount of dissolved substances contacted with a given surface area and not, within reason, by the concentration of dissolved substances in the inhibitor solution. In general, contacting the surface with a relatively concentrated solution for a short period of time is equivalent to contacting the surface with a dilute solution for a larger period of time, provided equivalent amounts of dissolved substances are contacted with a given surface area in each instance. Thus, the concentration of zinc ion and chromate ion in the inhibitor solution may vary over wide ranges, it being understood that for the purpose of the specification and claims the zinc salt and source of chromate ion are considered to be completely ionized. Inasmuch as the solution is applied in such quantity as to be in intimate contact with the hot zinc surface for a very short period of time and is then volatilized substantially immediately, better results will be obtained at higher temperatures when the solution is relatively dilute, while at lower temperatures the solution may be more concentrated. When operating at a temperature of about 225 F. to 600 F., the concentration of zinc ion and chromate ion preferably should total about one gram ion. In addition, for preferred results, the volume of solution which is applied to each square foot of zinc surface treated should contain initially a total gram ion amount of about 4.3 10- to 3.5 X 10- mole of zinc ion and chromate ion. When such quan tities of solution are applied to the hot zinc surface under the conditions taught herein, then adequate corrosion inhibition will result and a satisfactory corrosion inhibited product is produced.

When practicing the present invention, the zinc surface to be treated may be heated to the desired temperature and then intimately contacted with the atomized solution.

The volatile components of the inhibitor solution are substantially instantaneously evaporated leaving behind a thin invisible colorless corrosion inhibiting film deposited on the zinc surface. The corrosion inhibited zinc surface does not require further treatment and generally further treatment such as a water wash is not preferred.

The solvent of the inhibitor solution may be a suitable volatile solvent in which the zinc salt and source of chromate ion are sufficiently soluble. In. addition, the. boiling point of the particular solvent selected should. be such as. to allow application of the solution to the hot zinc surface to be treated in accordance with the invention. Water is the most satisfactory solvent when the .zinc salt is either zinc chloride or zinc nitrate and the source of chromate ion is chromic acid. However, alcohol, alcohol. water mixtures, orother suitable solvents will give satisfactory results under proper conditions. When water is, the solvent, water hardness is notdetrimental many great extent. Mildly hard water alters the inhibiting results only slightly, but somewhat :better results generally will be obtained by the use of distilled water or deionized water and particularly where the water supply is extremely hard.

The zinc surface to be treated should be given a suitable conventional pretreatment for the removal of oils, greases, etc., where such are present. However, the presence of a thin film of oxide on the zinc surface is usually not detrimental to the process of the present invention and, in fact, often the presence of a thin oxide film is beneficial. In instances where considerable corrosion has taken place, it may be necessary to remove the major amount of corrosion. In general, freshly galvanized sur-. faces do not exhibit such a degree of corrosion as to. necessitate removal of oxide film in a pretreatment.

While the process of the present invention may be used at any suitable time, it is preferred that the zinc surface be treated shortly after the zinc surfaced material or article has been manufactured. In some instances, the treatment may be successfully incorporated in the manufacturing process. For example, it is possible to incorporate the present invention in a conventional continuous galvanizing line for ferrous metal strip at any suitable stage after application of the zinc coating, provided the proper temperature conditions are met. This may be done by first cooling the molten zinc on the steel base to a temperature sufiicient to solidify the same but still allow the proper temperature conditions to be met. Then, following the cooling step, the inhibitor solution is sprayed on the hot galvanized sheet. Inasmuch as the solution is substantially instantaneously evaporated, and since further treatment of the strip is not necessary or evendesirable in most instances, the corrosion inhibited galvanized steel strip is then ready for coiling or packaging in the usual manner.

Corrosion inhibiting films deposited in accordance with the present invention are invisible to the naked eye, colorless in the amounts applied, and do not noticeably change the appearance of the zinc surfaced material. The resulting film is tenaciously adhered to the zinc surface in a manner suggesting chemical bonding and thus is to be distinguished in this respect from prior art chemical films. The films deposited in accordance with the present invention have been found to be very resistant to leaching and toexhibit corrosion inhibition properties over much longer periods of time than conventional chemical treatments.

In general, once a minimum amount of corrosion inhibiting film is deposited, thinner films are preferred over thicker films. For example, each square foot of surface area should .be intimately contacted with a volume of solution containing a total amount between about 4.3)( and 3.5x 10- gram ion of zinc ion and chromate ion. If greater quantities of solution are used, then the corrosion inhibiting value of the film tends to deteriorate for some reason unknown at the present time and the best results generally are not obtained. If amounts of film are deposited smaller than as above specified, thenin most instances a sufficient degree of corrosion inhibition is not imparted to the zinc surface to result in a superior product.

The manner in which the process of the present invention results in improved inhibition of white rusting is not fully understood at the present time. However, the following illustrative over-all reaction is presumed to take place when-the solution contains zinc chloride and chromic acid:

It is thought that some zinc dichromate also may be formed and deposited along with the zinc chromate, and perhaps still other species of chromate may be deposited. The reaction to form zinc chromate when operating in accordance with the present invention is not believed to occur until the solution is in contact with the hot zinc surface. The zinc chromate so formed and under the conditions of' the reaction then appears to react with or to be attracted to the zinc surface in some manner thereby causing the deposited thin invisible film to tightly adhere to the zinc surface in a manner suggesting chemical bonding. Whatever the theory, the process of the present invention is highly effective in inhibiting white rusting of zinc surfaces.

Although the present invention has been specifically described herein in connection with contacting an atomized solution with hot zinc surfaces, other equivalent means of application may be used. It is only necessary that the inhibitor solution be applied on the hot surface in a manner which permits the substantially immediateevaporation of volatile components. Thus, the solution may be applied by a roll or other suitable means as a thincontinu- .ous film as distinguished from a finely divided atomized spray.

The foregoing detailed description and the following specific examples are for the purpose of illustration only and are not intended as limiting to the spirit or scopev of the appended claims.

. EXAMPLE I The various samples treated and tested in accordance with the procedures of this example were prepared from galvanized steel sheet. The sample size was about 4 x 4 inches.

Each sample was'treated and tested in accordance with the following general procedure:

.(.1). The sample was heated 10 minutes a mufiie furnace at 600. F.

(2) The sample was removed from the mufilefurnace and immediately sprayed for varying periods of time (2, 4, 6, 8, 15 seconds) with aqueous test solutions of varying composition (solutions 1-7).

(3) The sample spray gun distance was 10 inches. and the spray gun pressure was 10 p.s.i., using pressurized argon as-the gas source. A standard type spray gun was used and under such operating conditions the average particle size in the spray was about l0l50 microns.

(4) The treated sample was. placed over tepid Walter and observed at intervals of 1 and 7 days. Where the sample exhibited substantially no visible white-rusting, the results were recorded as Good (G). If a smallamounvt of white rusting was visible, or if considerable white rusting was visible, then the results were recorded as Fair (F) and Bad (B), respectively.

The aqueous solutions tested had the following compositions, with all percentages being by weight:

Composition Solution No.

The volume of solution sprayed on each 10 square inches of surface treated was determined experimentally tobe 0.024 ml. when. the spraying time was 4 seconds. Volumes of solution. for spraying times other than 4 seconds may' be readily calculated from this data.

In all instances, where auwhite rust inhibition value of 7 "Good was obtained, volatile components of the atom ized solution evaporated almost instantaneously after contacting the heated surface and a thin invisible corrosion inhibiting film was deposited on the heated surface. This deposited film was found to adhere to the metal surface with a tenacity suggesting chemical bonding and could not be readily leached. The samples were not discolored or otherwise adversely affected in appearance after treatment.

The results of treating and testing various samples in accordance with the above described procedure are tabulated below in Table I.

Table I WHITE Russ. INHIBITION Spraying Time (sec) 2 sec. 4 sec. 6 see. 8 sec. 15 sec.

Solution No.

Exposure Time (days) 25- 7 F 26.-- 7 G G 27 Distilled Bad after 1 day exposure Water ZB-bad; F-fair; G-gcod. 7

It may be observed from the above data that the most satisfactory degree of white rust inhibition is obtained when using either Solution 1 or 2 and for minimum spraying time, in the case of Solution 2. In general, once a minimum amount of film to effect corrosion inhibition is deposited, then thinner films give more satisfactory results than heavier films. When using either Solution 1 or 2, a spraying time of 4 seconds is preferred in most instances.

EXAMPLE II Samples were prepared, treated and tested following the procedure of Example I unless otherwise indicated below.

A series of test solutions having a total molar amount of zinc chloride and chromic acid of 1.0 mole were prepared. The concentrations of zinc chloride and chromic acid in the solutions were varied between 1.0 mole zinc chloride, 0.0 mole chromic acid and 0.0 mole zinc chloride, 1.0 mole chromic acid. The solutions thus prepared were sprayed on the samples using the optimum spraying time of 4 seconds. The treated samples were then tested. The results are illustrated in the drawing.

Upon reference to the drawing, it will be noted that a white rust inhibiting value of Good" is obtained when the molar ratio of zinc chloride to chromic acid is between about 9:1 and 1:95, with even better results being obtained with molar ratios of zinc chloride to chromic acid between about 4:1 and 1:9. In most instances, a solution containing about equimolar amounts of zinc chloride (Zn++) and chromic acid (CrO will give superior results and is preferred.

8 What is claimed is:

1. A process for treating material having a zinc surface to improve corrosion resistance comprising bringing a solution consisting essentially of at least one compound which is a source of zinc ion and at least one compound which is a source of chromate ion dissolved in a volatile solvent into intimate contact with the zinc surface of the material, the solution being free of a substance which is a reducing agent for chromate ion and containing a ratio of zinc ion to chromate ion of 9:1 to 129.5, the zinc surface to be contacted with the solution being initially at a temperature substantially above the boiling point of the solvent and below the melting point of zinc, the volume of solution contacted with the zinc surface being controlled and the temperature of the zinc surface being sufiiciently elevated so as to volatilize the volatile components of the solution substantially immediately after contact and produce a substantially colorless corrosion resistant film on the zinc surface.

2. The treated material of improved corrosion resistance produced by the process of claim 1.

3. A process for treating material having a zinc surface to improve corrosion resistance comprising intimately contacting the zinc surface of the material with a solution consisting essentially of at least one compound which is a source of zinc ion and at least one compound which is a source of chromate ion dissolved in water, the solution being free of a substance which is a reducing agent for chromate ion and containing a ratio of zinc ion to chromate ion of 9:1 to 1:95, the zinc surface to be contacted with the solution being initially at a temperature from 225 F. to 600 F., the volume of solution contacted with the zinc surface being controlled and the temperature of the zinc surface being sufiiciently elevated so as to volatilize the volatile components of the solution substantially immediately after contact and produce a substantially colorless corrosion resistant film on the zinc surface.

4. A process for treating material having a zinc surface to improve corrosion resistance comprising bringing a solution in a volatile solvent of chromic acid and at least one zinc salt selected from the class consisting of zinc chloride and zinc nitrate into intimate contact with the zinc surface of the material, the solution containing a molar ratio of the zinc salt to chromic acid of 9:1 to 1:95, the zinc surface to be contacted with the solution being initially at a temperature substantially above the boiling point of the solvent and below the melting point of zinc, the volume of solution contacted with the zinc surface being controlled and the temperature of the zinc surface being sufficiently elevated so as to volatilize the volatile components of the solution substantially immediately after contact and produce a substantially colorless corrosion resistant film on the zinc surface.

5. The process of claim 4 wherein the solution contains initially a molar ratio of zinc salt to chromic acid between 4:1 and 1:9.

6. The process of claim 4 wherein the total molar amount of zinc salt and chromic acid initially present in the volume of solution contacted with each squ re foot of zinc surface area is between about 4.3 l0- and 3.5 X 10* mole.

7. The process of claim 6, wherein the solution contains initially a molar ratio of zinc salt to chromic acid between 4:1 and 1:9.

8. A process for treating material having a zinc surface to improve corrosion resistance comprising intimately contacting the zinc surface of the material with an aqueous solution of zinc chloride and chromic acid. the solution containing a molar ratio of zinc chloride to chromic acid of 9:1 to 129.5, the zinc surface to be contacted with the solut on being initially at a temperature from 225 F. to 600 F. the volume of solution contacted with the zinc surface being controlled and the temperature of the zinc surface being sufliciently elevated so as to volatilize the volatile components of the solution substantially immediately after contact and produce a substantially colorless corrosion resistant film on the zinc surface.

9. The process of claim 8 wherein the temperature of the zinc surface is from 250 F. to 350 F.

10. The process of claim 8 wherein the solution contains initially a molar ratio of zinc chloride to chromic acid between 4:1 and 1:9.

11. The process of claim 8 wherein the total molar amount of zinc chloride and chromic acid initially dissolved in thevolume of solution contacted with each square foot of zinc surface area is between about 4.3 10- and 3.5 x 10- mole.

12. The process of claim 10 wherein the temperature of the zinc surface is from 250 F. to 350 F.

13. The process of claim 11 wherein the temperature of the zinc surface is from 250 F. to 350 F.

14. The process of claim 11 wherein the solution contains initially a molar ratio of zinc chloride to chromic acid between 4:1 and 1:9.

15. The process of claim 14 wherein the temperature of the zinc surface is from 250 F. to 350 F.

16. A process for treating material having a zinc surface to improve corrosion resistance comprising intimately contacting the zinc surface of the material with an aqueous solution of zinc nitrate and chromic acid, the solution containing a molar ratio of zinc nitrate to chromic acid of 9:1 to 1:95, the zinc surface to be contacted with the solution being initially at a temperature from 225 F. to 600 F., the volume of solution contacted with the zinc surface being controlled and the temperature of the zinc surface being sufliciently elevated so as to volatilize the volatile components of the solution substantially immediately after contact and produce a substantially colorless corrosion resistant film on the zinc surface.

17. The process of claim 16 wherein the temperature of the zinc surface is from 250 F. to 350 F.

. 18. The process of claim 16 wherein the solution contains initially a molar ratio of zinc nitrate to chromic acid between 4:1 and 1:9.

19. The process of claim 16 wherein the total molar amount of zinc nitrate and chromic acid initially dissolved in the volume of solution contacted with each square foot of zinc surface area is between about 4.3 x10 and 3.5 X 10'- mole.

20. The process of claim 18 wherein the temperature of the zinc surface is from 250 F. to 350 F.

21. The process of claim 19 wherein the temperature of the zinc surface is from 250 F. to 350 F.

22. The process of claim 19 wherein the solution contains initially a molar ratio of zinc nitrate to chromic acid between 4:1 and 1:9.

23. The process of claim 22 wherein the temperature of the zinc surface is from 250 F. to 350 F.

24. A process for treating material having a zinc surface to improve corrosion resistance comprising intimately contacting the zinc surface with an atomized aqueous solution of approximately equimolar amounts of chromic acid and at least one zinc salt selected from the class consisting of zinc chloride and zinc nitrate, the zinc surface to be contacted with the solution being initially at a temperature from 225 F. to 600 F., the volume of solution contacted with the zinc surface being controlled and the temperature of the zinc surface being sufficiently elevated so as to volatilize the voltile components of the solution substantially immediately after contact and produce a substantially colorless corrosion resistant film on the zinc surface.

25. The process of claim 24 wherein the temperature of the zinc surface is from 250 F. to 350 F.

26. The process of claim 24 wherein the total molar amount of chromic acid and zinc salt initially dissolved in the volume of solution contacted with each square foot of zinc surface area is between about 4.3)(10' and 3.5 X 10- mole.

27. The process of claim 26 wherein the temperature of the zinc surface is from 250 F. to 350 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,206,064 Thompson et al. July 2, 1940 2,210,850 Curtin Aug. 6, 1940 2,393,943 Thomas et a1. June 29, 1946 2,445,155 I Saukaitis July 13, 1948 2,559,812 Watson July 10, 1951 2,763,569 Bradstreet et a1. Sept. 18, 1956 2,846,342 Curtin Aug. 5, 1958 2,851,386 Hartman Sept. 8, 1958 2,904,413 'Hampel Sept. 15, 1959 FOREIGN PATENTS 570,054 Great Britain June 20, 1945 

1. A PROCESS FOR TREATING MATERIAL HAVING A ZINC SURFACE TO IMPROVE CORROSION RESISTANCE COMPRISING BRINGING A SOLUTION CONSISTING ESSENTIALLY OF AT LEAST ONE COMPOUND WHICH IS A SOURCE OF ZINC ION AND AT LEAST ONE COMPOUND WHICH IS A SOURCE OF CHROMATE ION DISSOLVED IN A VOLATILE SOLVENT INTO INTIMATE CONTACT WITH THE ZINC SURFACE OF THE MATERIAL, THE SOLUTION BEING FREE OF A SUBSTANCE WHICH IS A REDUCING AGENT FOR CHROMATE ION AND CONTAINING A RATIO OF ZINC ION TO CHROMATE ION OF 9:1 TO 1:9.5, THE ZINC SURFACE TO BE CONTACTED WITH THE SOLUTION BEING INITIALLY AT A TEMPERATURE SUBSTANTIALLY ABOVE THE BOILING POINT OF THE SOLVENT AND BELOW THE MELTING POINT OF ZINC, THE VOLUME OF SOLUTION CONTACTED WITH THE ZINC SURFACE BEING CONTROLLED AND THE TEMPERATURE OF THE ZINC SURFACE BEING SUFFICIENTLY ELEVATED SO AS TO VOLATILIZE THE VOLATILE COMPONENTS OF THE SOLUTION SUBSTANTIALLY IMMEDIATELY AFTER CONTACT AND PRODUCE A SUBSTANTIALLY COLORLESS CORROSION RESISTANT FILM ON THE ZINC SURFACE. 